The following background information may present examples of specific aspects of the prior art (e.g., without limitation, approaches, facts, or common wisdom) that, while expected to be helpful to further educate the reader as to additional aspects of the prior art, is not to be construed as limiting the present invention, or any embodiments thereof, to anything stated or implied therein or inferred thereupon.
It is known in the art that tensile strength of soft rock and soils describes the capacity of the tested material to resist tensile stress as generated by external tension loads. At present, the test methods for soil and soft rock are mainly uniaxial tension method, Brazilian splitting method, axial fracturing method and soil beam bending method. Therein, the uniaxial tension method is a direct test method, and the Brazilian splitting method, the axial fracturing method, and the soil beam bending method are indirect test methods.
The uniaxial tension method is the direct application of tension at the two ends of the sample until the sample is broken. The tensile strength is calculated by the peak tension force divided by the area of the fracture section at the time of fracture. With the uniaxial tension method, in the test, the complete process towards failure of the sample can be observed, which can intuitively reflect the mechanical behavior of the sample under tension. But, the uniaxial tension process has difficulties in mounting the sample of soil and soft rock materials. At present, the uniaxial tension test often uses a clamp or a polymer glue to connect and fix the two ends of the cylindrical sample to the device. The fixing clamp is of two semi-circular jaws, and screws are used to buckle and fasten the two jaws so as to fix the two ends of the sample. Because the soil and soft rock are generally weak and brittle, the ends of the sample are easily damaged under pressure when being fixed with fixing clamp or polymer glue. Secondly, it is difficult to center the clamps at the two ends of the sample, so that eccentric force can be easily generated during the test, which affects the test result. Furthermore, the relative displacement between the clamp and the sample is easy to occur in the process of tension, so that it is difficult to measure the true value of the deformation of the sample. With the use of polymer glue to bond the ends of the sample, the internal structure of the ends of the sample is easily damaged, the sample and the device are difficult to be bonded, and it is easy for the sample and the device to get detached in the process of tension.
The indirect test methods assume that the tensile failure of soil or soft rock follows a certain stress-strain relationship, and the tensile strength of the material is indirectly calculated by using a theoretical formula matching the test method. The specification of the Brazilian splitting method requires that the padding strips must be placed symmetrically above and underneath the sample, and requires that both of the contact lines between the two padding strips and the sample should be on the vertical central section of the sample. But, because the padding strips are small in size, it is difficult to meet the above requirements, so that eccentric pressure occurs when pressure is applied. In addition, the padding strips are likely to move in the process of pressure application, which results in failure of the test.
The fracturing effects of the sample in the axial fracturing test are not easy to control, and because it is difficult to center the upper and lower small-sized cylindrical blocks in the center axis of the sample. The sample as needed for the soil beam bending method is relatively large in size and is difficult to prepare, and it is difficult to make sure that the fracture position of the sample occurs along the middle cross section of the sample.
In addition to the aforesaid existing problems, the load in the current tensile strength test methods is generally provided by a universal testing machine, and the related parts such as clamps, padding strips and cylindrical blocks need to be manufactured and customized additionally, so that the entire test system is expensive in price, and bulky in size. The test requires multiple people to cooperate, the operation is complicated with trivial details, and the efficiency is low. The test involves many steps of manual operation, and it is difficult to achieve precise placement of parts, which affects the final results of the test.